The invention relates to a method and apparatus for the plasma treatment of rod-like or thread-like materials.
The surface of several materials, in particular synthetic resins, cannot, or only with difficulties, be wetted with liquid. When a print, lacquer or an, adhesive layer shall be applied to such materials, a pretreatment of the surface thereof is therefore normally required.
A conventional method to enhance the capability of plastics surfaces to be wetted consists of applying a corona discharge to the surface. In contrast, European Document 0 761 41 5-A2 discloses a method in which the pretreatment is not performed by means of corona discharge but by means of a low temperature plasma under atmospheric pressure. A plasma nozzle for performing this method is also disclosed in the cited document. During pretreatment, the plasma nozzle is directed onto the surface to be treated, so that the plasma jet scans the surface like a painting brush.
The materials to be treated can also be rod-like or thread-like materials such as synthetic fibers, yarns, threads, wires, plastic-coated cables, glass fibers and the like.
It is an object of the invention to provide a method and apparatus with which a particularly efficient plasma treatment of such rod-like or thread-like material is made possible.
According to the invention, there is provided a method for plasma treatment of rod-like or thread-like material, wherein the material is passed coaxially through a plasma nozzle.
According to another aspect of the invention, an apparatus for treating rod-like or thread-like material comprises a plasma nozzle having a nozzle pipe which forms an outer electrode and an inner electrode disposed coaxially in said nozzle pipe, wherein a passage is formed coaxially in said inner electrode for passing the material to be treated therethrough.
Typically, a plasma nozzle which forms the essential part of the apparatus for performing the method according to the invention comprises a nozzle pipe having an outlet end and forming an annular or tubular outer electrode at least at said outlet end. A working gas flows through this nozzle pipe. A stud-shaped inner electrode is formed coaxially inside of the nozzle pipe. The plasma is generated by an electric discharge between the inner and outer electrodes. To this end, a high voltage, preferably an AC voltage with high frequency, is applied to the electrodes. A generator for providing this voltage is described in German Document No. 42 35 766.
In the apparatus according to the invention, the inner electrode of the plasma nozzle is formed with a coaxial passage, and the rod-like or thread-like material to be treated passes through this passage. Thus, the material passes centrally and coaxially through the plasma flame jetted out from the plasma nozzle, and the entire peripheral surface of the material can evenly be treated in a single process.
The method according to the invention is also suitable for plasma polymerization or plasma coating of the material. In the latter case, the coating material is supplied in a gaseous state together with the working gas or via separate nozzles disposed at appropriate positions.
Preferably, the working gas is swirled in the nozzle pipe. This imports also a swirling movement to the plasma flame jetted out from the nozzle so that the plasma flows around the material in vortex fashion and is thereby brought in intimate contact with the surface of the material.
At least in those cases in which the material to be treated is not electrically conductive, the material can freely pass through the nozzle pipe over a substantial portion of its length. By swirling the working gas, a vortex is created in the nozzle pipe, and the arc of the electric discharge Is channelled inside of the nozzle pipe in the core of this vortex, so that it will not fan-out to the surrounding outer electrode before it reaches the outlet end of a nozzle. When the material passes through the nozzle pipe, the electric are channelled in the manner described above will wind helically around the material, without however contacting the same. Thus, it is also possible to achieve a thermal treatment of the surface of the material, the intensity of this treatment being controllable by controlling the swirling movement of the working gas and/or by supply of a secondary gas.
In case of certain materials, in particular materials having a relatively rough or fluffy surface such as staple fibers, leak air can be entrained when the fiber passes with high velocity through the passage formed in the inner electrode. This effect, by which the action of the plasma or the thermal action may possibly be deteriorated, can be avoided or mitigated by creating a subatmospheric pressure in the passage of the inner electrode.